Generally, a liquid crystal display mostly uses a light bar having a number of evenly spaced light emitting diode (LED) as a backlight. A huge number of light bars with different intervals of two LEDs needs to be manufactured in order to reduce the number of LEDs and decrease the distance between the light bar and the light guide plate under a condition that uneven brightness of backlight due to a gap between two LEDs can sanctify a predetermined quality requirement, which leads to a high testing cost and is not easy to find out a maximum critical value of LED interval. Otherwise, the distance between the light bar and the light guide plate would be changed when the light bar is replaced. It is difficult to make sure a precision of the distance even if a measuring instrument is used to regulate the distance.
Therefore, an optical testing device which can solve the above-mentioned problem needs to be provided.